The Islamic State fighters, including an American jihadist, were reportedly killed near the group's stronghold of Raqqa

* Foreign fighters killed include American, says monitoring group

* Assad's forces step up aerial attacks

By Suleiman Al-Khalidi

AMMAN, Sept 5 (Reuters) - Eighteen foreign fighters from the Islamic State, including an American jihadist, were killed in a Syrian air raid on a town near the militant group's main stronghold city of Raqqa in eastern Syria, a human rights monitoring group said on Thursday.

The Syrian Observatory for Human Rights, which has tracked violence on all sides of the three-year-old conflict, said reliable sources reported that top Islamic State leaders who happened to be in the municipal building of Gharbiya at the time of the raid were among the foreign fighters killed.

The building had been used as a headquarters of the hardline group, according to the monitoring body.

Another air raid on Thursday that hit a former intelligence headquarters in the city of Abu Kamal near the border with Iraq that was used by the Islamic State also killed an undisclosed number of their members, the monitoring group said.

It said the Syrian raids allowed 13 detainees held by the IS fighters to escape during the chaos.

Proclaiming a 'caliphate' straddling parts of Iraq and Syria, Islamic State has swept across northern Iraq in recent weeks, prompting the first U.S. air strikes in Iraq since the withdrawal of American troops in 2011.

The insurgents are also tightening their grip in Syria, of which they now control roughly a third, mostly rural areas in the north and east.

Another opposition source in Raqaa said that two raids on southern and northern parts of the city of Raqqa on Thursday caused several civilian deaths and injuries.

The Syrian army has intensified aerial bombardment of rebel held areas in rural north western Syria in recent days including in the countryside of the city of Hama, where rebels have made some inroads and taken over checkpoints and towns, according to activists.

President Bashar al Assad's forces have stepped up an air blitz in the eastern suburbs of Damascus to recapture areas that have been in the hands of rebels for over a year, causing dozens of mainly civilian deaths, according to activists.

Rebels who claim to have repelled army attacks and killed dozens of army forces have endured ongoing ground shelling by government missile batteries located in Damascus' city centre. The shelling and heavy aerial bombardment have razed large residential areas to the ground in what activists say is a scorched earth policy.

Although the authorities have made gains in areas around the capital this summer, including the town of Mleiha just outside Damascus on August 14, rebels say they have succeeded in repelling further advances.

They say even the areas that fall to the army continue to witness ongoing clashes and mortar attacks by rebels positioned in neighbouring districts.

Although insurgents have been prevented from taking central Damascus, President Bashar al-Assad's forces are worried they will reach it by digging tunnels from the sprawling suburbs and outlying towns under their control.

Militants active there include the al Qaeda affiliated Jabhat al Nusra along with various Syrian rebel brigades.

Islamist fighters battling the Syrian army, who last week overran a U.N.-controlled crossing point on the "disengagement line" that separates Israelis from Syrians on the Golan Heights, said they had taken over strategic hills that could help them open new supply lines to tracts in southern Syria held by rebels.

The Syrian army has denied allegations of new rebel advances. (Reporting by Suleiman Al-Khalidi; editing by Andrew Hay)